A visible light source having strong monochromaticity and capable of outputting a W (watt)-class high output is required in realizing a large-size display, a high-luminance display and the like. For a red light source out of three primary colors of red, green and blue, a red high-output semiconductor laser used in a DVD recorder or the like can be utilized as a small-size light source with high productivity. However, it is difficult to realize a green or blue light source by a semiconductor laser or the like, wherefore small-size green and blue light sources with high productivity are being demanded. Particularly, it is highly difficult to realize a green light source since there is no suitable material used for a semiconductor laser.
A wavelength converter as a combination of a fiber laser and a wavelength conversion element is realized as a low-output visible light source of such a type. Green and blue small-size light sources using a semiconductor laser as a light source of excitation light for exciting a fiber laser and a nonlinear optical crystal as a wavelength conversion element are well known.
However, in order to obtain green and blue lights of W-class high output from such a wavelength converter, several problems need to be solved. FIG. 16 shows the schematic construction of a conventional wavelength converter 20. There is described a case where a green output light is, for example, obtained by this construction. The wavelength converter 20 shown in FIG. 16 is provided with a fiber laser 15 for outputting a fundamental wave, a wavelength conversion element 1 for converting the fundamental wave into a green laser light and a lens 2 for condensing a fundamental wave output on an end surface of the wavelength conversion element 1.
Next, a basic laser operation of the fiber laser 15 is described. First of all, an excitation light from an excitation laser light source 3 is incident on one end 4a of a fiber 4 in FIG. 16. After the incident excitation light is absorbed by a laser active material contained in the fiber 4, a seed light of the fundamental wave is generated in the fiber 4. This seed light of the fundamental wave reciprocates many times in a laser resonator using a fiber grating 4b formed in the fiber 4 and a fiber grating 5b formed in a fiber 5 different from the fiber 4 as a pair of reflection mirrors. Simultaneously, the seed light is amplified with a gain given by the laser active material contained in the fiber 4 to increase its light intensity and is also wavelength-selected to reach a laser oscillation. The fibers 4, 5 are connected by a connecting portion 6, and the laser light source 3 is current-driven by an excitation laser current source 7.
Next, a basic operation of the wavelength converter 20 is described. The fundamental wave is outputted from the fiber laser 15 as described above and incident on the wavelength conversion element 1 via the lens 2. The fundamental wave from the fiber laser 15 is converted into a harmonic by the nonlinear optical effect of the wavelength conversion element 1. This converted harmonic is partly reflected by a beam splitter 8, but the transmitted harmonic becomes a green laser light as an output light of the wavelength converter 20.
The harmonic partly reflected by the beam splitter 8 is utilized by being converted into an electrical signal after being received by a light receiving element 9 for monitoring the output light of the wavelength converter 20. An output controller 10 regulates a drive current of the laser light source 3 using the excitation laser current source 7 such that the intensity of this converted signal becomes an intensity to give a desired output of the wavelength converter 20. Then, the intensity of the excitation light from the laser light source 3 is adjusted and the output intensity of the fundamental wave of the fiber laser 15 is adjusted, with the result that the output intensity of the wavelength converter 20 is adjusted. In this way, a so-called automatic power control (hereinafter, abbreviated as “APC”), in which the output intensity of the wavelength converter is kept constant, stably operates.
It is possible to obtain a green high-output laser light of several 100 mW by such a construction, but it is difficult to obtain a W-class green high-output laser light. In other words, the outputs of the fundamental wave of the fiber laser and the excitation light need to be increased in order to increase the light output of the wavelength converter. However, in the construction of the conventional fiber laser shown in FIG. 16, the absorption of the fundamental wave increases in proportion to the fiber length even if it is tried to increase the gain of the fundamental wave by increasing the length of the fiber. Thus, if the fiber length exceeds a certain length, the fiber laser does not reach a laser oscillation any longer.
The absorption of the fundamental wave by the fiber notably increases as the oscillation wavelength of the fiber laser decreases. FIG. 17 shows an optimal fiber length in relation to the oscillation wavelength of the conventional fiber laser. AS the oscillation wavelength decreases from 1080 nm to 1020 nm, the optimal fiber length also decreases to shorten a section where the seed light is amplified, wherefore the output of the fundamental wave obtained from the fiber laser decreases.
In order to increase the output of the fundamental wave in such a situation, the output of the excitation light is increased to excite the fiber laser by setting the fiber length to a suitable length. However, since the fiber length is not sufficiently long, the excitation light cannot be completely absorbed, leaving the excitation light of the considerable output intensity. Accordingly, in the case of obtaining a W-class high output from such a wavelength converter using the fiber laser, a problem of deteriorating the fiber notably occurs due to a temperature increase or the like caused by an increase of the light output as the sum of the fundamental wave and the excitation light in the fiber and an increase of the light absorption. There is also a problem of damaging the excitation laser light source by the increased return light of the fundamental wave resulting from the increase of the light output in the fiber.
Various measures are taken to solve these problems. An example of preventing the damage of a laser light source, a wavelength selection filter is inserted in a light path so that a signal light amplified in a fiber and having a high peak light output does not return to an excitation laser light source in a fiber laser for optical communication. By utilizing a slight difference between an excitation light and the signal light, this wavelength selection filter reflects the signal light although transmitting the excitation light. Thus, only the excitation light is emitted from the excitation laser light source and no amplified signal light returns, wherefore the laser light source is not damaged (see, for example, patent literature 1).
On the other hand, there is an example of elaborating the structure of a fiber doped with a rare earth element as a laser active material in order to prevent the deterioration of a fiber. Specifically, in a fiber laser for outputting a fundamental wave having a wavelength in a 3 μm band used in the medical field, the absorptance of the excitation light is optimized by determining the range of the doped amount of the rare earth element and by adjusting the diameters of a core and a cladding of the fiber within specific ranges. By adopting such a structure, a high light output of 3 W is obtained without deteriorating the fiber laser (see, for example, patent literature 2).
In an optical communication field or the like, a connection structure of a fiber and a guide fiber is elaborated to prevent the deterioration of the fiber when an excitation light is incident on the fiber, to which a signal light is transmitted, via the guide fiber (see, for example, patent literature 3).
The deterioration of a fiber is also prevented by surrounding a core part of the fiber by an outer core made of a material having a higher refractive index than the core part and by leading the light to the outside of the fiber to suppress an increase of the light output if the light output in the fiber has increased (see, for example, patent literature 4).
In order to solve a problem different from that of the present invention, there has been also proposed a construction for reflecting an excitation light in a fiber to separate an oscillating light and an excitation light (see, for example, patent literature 5).
However, in the above conventional wavelength converters, the light output of the fiber laser as the fundamental wave is only 2 to 3 W and it is difficult to obtain an output of the fundamental wave exceeding 5 W. Thus, green and blue laser lights of W-class high output cannot be obtained. Even if the fiber length of the fiber laser is simply increased for the purpose of increasing the output of the fundamental wave by increasing the gain of the fiber laser, the absorption of the fundamental wave by the fiber increases in proportion to the fiber length, wherefore a large light output cannot be obtained.
On the other hand, since the absorption of the fundamental wave by the fiber becomes more notable as the oscillation wavelength of the fiber laser decreases from 1070 nm, the fiber length needs to be decreased to obtain a light of 1070 nm or shorter. However, if the fiber is shortened, efficiency decreases and it becomes difficult to obtain high efficiency, for example, in the case of producing a light having a wavelength of around 1030 nm.
Accordingly, there has been a problem of being difficult to obtain a W-class short wavelength green laser output light, which is supposed to be obtained by shortening the wavelength of the fundamental wave of the fiber laser.
Patent Literature 1:
                Japanese Unexamined Patent Publication No. H05-7038Patent Literature 2:        Japanese Unexamined Patent Publication No. 2005-79197Patent Literature 3:        Japanese Unexamined Patent Publication No. 2005-19540Patent Literature 4:        Japanese Unexamined Patent Publication No. 2004-170741Patent Literature 5:        Japanese Unexamined Patent Publication No. 2005-109185        